1. Field of the Invention
The present invention relates to an optical device, and an image forming apparatus.
2. Description of the Related Art
There have been utilized optical devices that include a light emitting element such as laser diode (LD) or light emitting diode (LED) to irradiate a target surface, and project an image from the target surface on various positions. Some image forming apparatuses, for example, copiers, printers, scanners, and facsimile machines, include such an optical device. In the optical devices and image forming apparatuses, if the performance is deteriorated by contamination of the surface of the optical element, treatments are usually performed to prevent contamination, to provide the optical element surface with contamination resistance, or to minimize performance deterioration even if the optical element is contaminated. If such treatments do not work well, contamination is removed or the optical element is replaced.
To prevent the contamination of the optical element surface or provide the optical element with the contamination resistance, a process is shut down that causes the contamination, a layout is provided that minimizes influences of gravity (a method of preventing horizontal layout), or a coating is applied that hardly attaches matters that can contaminate the surface.
To minimize the performance deterioration even if the optical element is contaminated, an image is enlarged on an optical path and focused on an irradiated surface, the amount of light is increased depending on the degree of contamination, or image processing is performed. Although there are many methods for removing contamination, such methods are not necessary if contamination is suppressed to a degree that the performance deterioration does not occur.
Meanwhile, electrophotographic devices with superior productivity and handling property become widely used as image forming apparatuses. Minimization of toner particles that influences greatly improvements in image quality has progressed. As toner particles have electric charges with a predetermined polarity because of their image forming function, they are easily attracted electrostatically. Therefore, measures against contamination of optical elements caused by toner are required.
For example, Japanese Patent Application Laid-Open No. H8-244277 discloses a conventional technology in which contamination of an optical system is prevented through the use of a power source that is effective for preventing contamination of charging wires and electrostatic absorption for attracting floating toners.
According to the conventional technology, contamination of an optical system is prevented by attracting floating toner around an optical element electrostatically. Toner that is actually floating in the image forming apparatus is usually moving not slowly but quickly to some extent because of airflow generated in the image forming apparatus.
Toner does not remain around the optical element but collides with the optical element to be attached thereto or removed therefrom. A few kilovolts of voltage must be applied to generate efficiently, on an electric field, an attraction force that is sufficient to detach once electrostatically attracted toner from the optical element by a predetermined spatial distance. Measures against leakage to the vicinity need to be also considered.